1. Field of the Invention
The present invention relates to a paper feeding cassette in a laser beam printer (LBP) using an electrophotographic developing method, and in particular, to a paper pickup device installed to keep plural sheets of paper in a pickup position.
2. Description of the Related Art
Electrophotographic development is used in printers, copiers, LBPs, facsimile machines and related devices. Such a device is equipped with a paper feeding cassette to accommodate plural sheets of paper, and the paper feeding cassette has a paper pickup device for keeping the paper pile in a pickup position.
Some examples of paper pickup devices of the conventional art are seen, for example, in the following U.S. patents. U.S. Pat. No. 4,793,606, to Yasuoka et al., entitled Paper Feeding Device for A Printer, discloses a paper feeding device for a printer which includes a paper feeding tray and a tray receiver. U.S. Pat. No. 4,830,353, to Hendricks et al., entitled Device For Holding a Stack of Sheets And Discharging Them From The Top Thereof, discloses a device for discharging sheets having a holder for a stack of sheets such that the stack is pressed against a conveyor roller disposed at a fixed location above the holder. U.S. Pat. No. 5,181,713, to Neugebauer, entitled Paper Tray For A Copying Machine, discloses a paper tray arrangement for receiving a stack of paper and which is insertable into a receiving device of a copying machine.
A paper pickup device is disclosed in a co-pending U.S. patent application Ser. No. 09/168,375 and a description of its structure is incorporated herein. In the mechanism of this device, with a subplate placed even, a first tension coil spring is strained between a first engaging portion of a connecting member and a shaft. The strained first tension coil spring applies a force to move a knock-up plate to a pickup position. The uppermost paper sheet of a paper pile on the knock-up plate is transferred by rotation of a pickup roller in close contact with the pickup roller.
Meanwhile, a second tension coil spring is kept stationary, that is, it is not strained. No tensile force is applied to the second tension coil spring because a slot is curved and the distance between a second engaging portion of the connecting member and an end of a lever is not changed by rotation of the connecting member around a hinge shaft. As a result, when the lever end is positioned in a portion of the slot, only the first tension coil spring is strained with pickup force, that is, the second tension coil spring is not strained.
When a paper pile is placed with a maximum load on a paper feeding cassette, user manipulation to rotate a locker around a hinge and position the lever end in the other portion of the slot increases the distance between the second engaging portion and the lever end Thus, the second tension coil spring is strained with a tensile force. That is, the user can freely strain the second tension coil spring by displacing the lever along the curved slot.
Here, the locker is fixedly positioned by a stopper and the force of the lever and the second tension coil spring In this state, the second tension coil spring is strained between the lever and the second engaging portion. It can be concluded that the user can strain the second tension coil spring by displacing the lever end along the slot and thus changing the distance between the second engaging portion and the lever. In addition, the shaft can be selectively positioned in two portions of a slot, which allows the user to freely adjust application of a paper pickup force along with the movement of the shaft.
However, the above paper pickup device suffers from the disadvantages that assembly is difficult and reliability of parts is decreased due to a large part tolerance involved in designing caused by requirement for many parts.